Surgery done on a single cell

Micromanipulation of the machinery of life

By William J. Cromie
Harvard News Office

Imagine a scalpel so sharp it can cut a connection between two nerve cells, or obliterate an invisibly small structure within a cell without harming any of hundreds of other structures floating close by. And it can do this without breaking the fragile surface of a cell.

Such a superprecise scalpel is now a reality thanks to experimenters at Harvard University. "Ultrashort laser pulses [up to 1,000 a second] produce a spot as hot as the sun," notes Eric Mazur, Gordon McKay Professor of Applied Physics. "Normally, that kind of heat would vaporize a cell, but it only shines for a millionth of a billionth of a second. The light intensity is very high, but the energy generated in such a short time can be compared to a mosquito bumping into your arm. A cell can easily take that."

A laser beam ordinarily travels right through a piece of glass or a transparent cell, but in this application it is focused into a very, very small space within a cell. "It's like lighting a hot spark inside the cell without disturbing the surface membrane, the fragile bag that holds the cell together," Mazur says.

An exacting technique like this opens up a plethora of medical possibilities. The Harvard researchers vaporized a single mitochondrion, a minute biological motor that provides power to a cell to carry out its many functions. They cleaved a single nerve in a tiny roundworm, knocking out the creature's sense of smell. Such experiments may some day lead to snipping connections in the human brain that are associated with disease and mental deficiencies. Or perhaps a fertilized egg cell can be tailored to set the gender of an offspring.

"We see this as a tool for the micromanipulation of life on a cellular level," says Mazur.

Probing cell skeletons

Mazur and his students didn't start out to build a laser that could change life. What they wanted at first was a beam that could burn cavities inside a piece of glass without breaking the glass. They constructed a system that sends a ruby red laser beam through a microscope to a focal point inside the glass. The beam passes through the transparent surface, getting narrower and narrower until it becomes a dot less than a millionth of an inch in diameter.

The laser blast lasts only a millionth of a billionth of a second, but it's intense enough to rip electrons off an atom. Such microscopic violence leaves behind a cavity that can be used for data storage. The technique has been patented by Harvard.

Subsequently, Mazur listened to a lecture by Donald Ingber, a professor of pathology at the Harvard Medical School who studies cell skeletons. The cell is not a shapeless bag of fluid, genes, and other parts; rather, it is given a shape by a delicate network of protein fibers thinner than a fine human hair.

Ingber was looking for a way to cut away discrete sections of these Lilliputian girders to determine their contribution to the functions and forces that rule a cell. Mazur told him, "I have just the tool for you."

"He was right," Ingber comments. "We were able to knock out a single unit, a mitochondrion, without vaporizing other units less than a millionth of an inch away. This is the kind of amazing precision that will enable us to determine how cells form, how they move, and how some molecules get from place to place in a cell." In some cases, molecules slide along the skeletal fibers like passengers riding on commuter railroads.

This work was done in the skin cells of mice. But, as Mazur points out, mouse cells are not that much different from human cells, just easier to handle in a laboratory.

Snipping smell out of a cell

"The results were so exciting," Mazur continues, "we decided to go beyond single cells to see if we could sever a connection between fibers connecting two nerve cells without vaporizing nearby cells and their connections. Mazur started collaborating with Avavinthan Samuel, a Harvard scientist with one leg in biology and the other in physics.

For their subject, they chose a worm so small it's hard to see. The creature never grows more than a few hundredths of an inch long, is as thin as a hair and, best of all, it's transparent. Called C. Elegans, the creature is, as Mazur says, "little more than a tiny tube with a mouth, gut, and anus." It consists of a mere 1,300 cells, as opposed to some 150 trillion in a human. About 300 of these cells control the worm's behavior, when and how it moves, feeds itself, and avoids noxious chemicals. Mazur and Samuel lased a single connection between two nerve cells, knocking out the worm's sense of smell, literally a mind-boggling bit of surgery.

Such experiments show that laser surgery is an exciting new way to learn the functions of all types of cells, most of which are transparent. Would it also be a way to treat disease and cellular defects? "It's too soon to tell," Mazur answers. "Someday, we might find some specialized use for snipping a nerve here and a cell element there, but right now the technique is confined to studying the machinery of life."

Mazur points out that humans are built of trillions of cells, rather than 1,300. "Cutting a connection or two is not going to have the same effect on a human that it has on a minute worm," he notes. However, he admits, superprecise laser surgery could influence nature in the important function of reproduction. If you changed something in a fertilized egg cell, that alteration would be propagated over and over as the egg divides again and again to form a bacteria, frog, or human.

"One cell modification might be enough to change an organism's gender," Mazur proposes. "I see that as perhaps the first major application of this technique."